When it comes to muscle cars, or super cars as they were known back then, few would argue that the 1970 Buick Gran Sport and GSX with the 455 Stage 1, were among the fastest and most powerful cars of their day. Bench racers across the land forever will argue which is the fastest of them all, but a January 1970 road test by Motor Trend magazine netted a 13.38 second elapsed time at 105.5 miles per hour with a new GSX. The car in question was running on Goodyear Polyglas GT G60-15 tires and featured the 455 engine in Stage 1 tune, rated at 360 hp and 510 lb-ft of torque (gross) along with a TH400 three-speed automatic and posi rear end with 3.64:1 final drive.

There’s no question that on paper, the specs of that car were nigh identical to our test victim seen here–the problem, of course, is the variables. Was the MT test car a ringer? What was the track temperature during that run? What kind of fuel did they use? What altitude were they racing at? How warm were the tires? How much timing did the engine have? Well, you get the picture.

POWER OUTPUTAnd it’s not only quarter-mile times that have become the holy grail of muscle car bench wars, it’s also the power output. The standard 1970 455 cubic inch Buick V-8, as fitted to the Gran Sport and GSX models that year, was rated at 350 horsepower (gross) at 4600 rpm and 510 lb-ft of torque (gross) at 2800 rpm. It featured a 4.3125-inch bore and 3.90 inch stroke. A cast-iron intake manifold, GM Rochester Quadrajet 4MV four-barrel carburetor and forged alloy pistons, and a 10.0:1 compression ratio were all standard. The Stage 1 Buick 455 V-8 boasted 10.5:1 compression, a camshaft with a hotter profile, more duration and increased lift, plus heads with unique (and larger) intake and exhaust valves. The result was 360 horsepower, at the same revs as the regular 455, while torque was unchanged. These improvements to the valvetrain were designed to improved top end breathing and hence horsepower, but in an engine like the Buick 455–whose primary purpose was to produce enough torque to yank big barges like Electras and Rivieras around–it seemed almost redundant. It’s often been said that Buick underrated power output of the Stage 1 455, perhaps in an attempt to ease insurance concerns. But considering that the entry price of a 1970 GSX (before options) was $4,479 and that Buick didn’t exactly promote it as an all-out street fighter, that’s probably a bit of a moot point.

THE PASSING OF TIMETrouble is, since those halcyon days of the original muscle car wars machines like this became used and abused, in many cases modified, and then ultimately over-restored and stashed in warehouses rarely to see the light of day. Today many of the remaining GSXs enjoy a leisurely life on the show circuit–seldom, if ever, getting the chance to stretch their legs. It’s one thing having a car that’s beautifully restored, it’s entirely another to have one that runs as good as it looks. And because many GSXs still in existence haven’t turned a wheel in anger in almost four decades (stock appearing drag racers aside), how do you know that when you buy one of these things, you’re getting your money’s worth?

That’s what we thought. So one day, Classic Cars & Parts decided to document a real Buick GSX on a Dynojet 248 rolling road to see just how much power this engine makes in factory tune–and where, if any, improvements can be had.

OUR VICTIMThe purpose of this exercise was to see what a concours restored 1970 GSX can really put out without resorting to a raft of aftermarket parts. The car has been frame off restored; to the point that the Delco battery, hose clamps, and Polyglas tires are correctly reproduced. The engine has also been rebuilt–all indications are to factory specs, but since completion the car has been little more than a trailer queen. Although the odometer currently displays 79,314 miles, this GSX has been driven less than 20 miles since the restoration was finished some three years ago, mostly in and out of warehouses and showrooms. A car like this can fetch over $100,000 on the open market.

THE TEAMTo make our test possible, we enlisted the help of the folks at Legendary Motor Car Company Ltd, based in Milton, Ontario Canada. Run by Peter Klutt, this is a world-class restoration, performance, and racing facility specializing in quality muscle cars. It’s also the backbone behind the long running SPEED channel TV show Dream Car Garage, hosted by Peter and Tom Hnatiw. If anybody was capable of seeing what a factory-trim ’70 GSX Stage 1 could do on a dyno, it was these guys.

After strapping the car down on the Dynojet at Legendary, we did a baseline run to see some numbers. Technician Jason Humphries rigged up the Dynojet software and fired up the big 455 V-8. Because it was cold outside, and there was less than 7 percent humidity, it was important to set the mixture to prevent a lean condition and the risk of destroying a rather expensive engine. Let it be noted that the engine’s crankcase had been filled with 10W40 mineral oil, the gas tank contained Sunoco Ultra 94 unleaded fuel, and a Crane electronic ignition and rev limiter had been installed on this car for better spark performance and improved engine safety. Beyond that everything was as close to the way Buick assembled it nearly 40 years ago.

Our baseline yielded 151.5 horsepower and 202.3 lb-ft of torque at the tires. To figure out the gross horsepower ratings, we calculated a 15 percent reduction from gross to SAE net horsepower and then a 20 percent loss through the car’s inefficient TH400 trans and the rest of the drivetrain. The result was 204.5 hp (gross) and 273.1 lb-ft of torque. It is also worth noting that the air/fuel ratio was at 13.0 parts air to 1 part of fuel (13.0:1) during our test run–the standard air/fuel ratio for peak power in naturally aspirated cars.

However, despite taking these precautions, it soon became obvious that the Stage 1 was running rough; the engine was missing and bogging during that first run. The problem was soon traced to a distributor that was loose, and the timing was retarded four degrees initial from the factory recommended advance. Additionally, during that first run the big 455 spewed out enough carbon and rust from the exhaust to fill a wheelbarrow.

After tightening up the distributor and setting the timing to 10 degrees initial, then waiting 30 minutes for the engine to cool, we fired her up again. The barometer readings had changed slightly, the humidity had crept up a percentage point (from 7 to 8) and the run temperature jumped a couple of degrees from 72.45 to 74.55°. Our second pull yielded a big improvement–182.6 hp and 276.71 lb-ft of torque to the G60 F-15 Polyglas tires. This time there was no miss and the engine ran much stronger and smoother, though interestingly power peaked at 3800 rpm and torque peaked at 2600, in both cases well below the factory rating of 4600 and 2800 revs, respectively. Calculating in our parasitic losses through engine accessories and the driveline, we calculated that on this second run, the Stage 1 engine was making 246.5 hp (gross) at 3800 rpm and 373.5 lb-ft (gross) at 2800. There was still a long way to go.

We were starting to get the feeling that our Stage 1 was an under achiever and in need of a good dragstrip run to flush out the carbon. Still, we wanted to see if waking her up a bit more would net any sizeable amount of improvement. After a 45-minute wait, we started the big V-8 once again. With no further adjustments to the carb or the distributor we tried again. The humidity had notched up another percentage point, but the ambient temperature had dropped back down, to 72.55° for this one. The results came in soon enough–223.5 hp at 4000-4600 rpm and 308.2 lb-ft at 2900-3500 rpm (a nice and flat torque curve). Clearly things were getting better. Factoring in our calculations, we arrived at 301.7 hp (gross) and 416.0 lb-ft of torque (gross). The dyno chart shows our second run in red and our third in green.

CONCLUSIONAn interesting observation was that on the third run, maximum power and torque delivery were consistent with the factory rpm settings, but although we felt there was still more power to be obtained, it was looking like horsepower and torque would ultimately be closer together than the factory ratings suggested–based on our own experience and calculations, in cold weather with minimal humidity. As a result it was possible to argue the point that perhaps the Stage 1’s factory horsepower rating was closer to being accurate after all and that the torque figure was perhaps a bit further off and somewhat less optimistic. On the other hand it was also still possible that a Stage 1 engine could actually make close to 400 hp in factory tune with torque in the 500 lb-ft range, given a little more time spent tweaking and running.

And while there’s clearly a lot more potential lurking under that pristine Saturn Yellow hood, even at this early stage (pun intended), one thing was already certain: the 1970 Buick GSX equipped with the Stage 1 455, whether trailer queen or weekend warrior, is truly one giant of a sleeper on street or track–and not a car to be messed with.

Tuning carbureted engines in classic vehicles can be difficult, but if you’re buying a restored muscle car, there are a number of things you can do to avoid costly mistakes, and ensure things go in your favor. Legendary’s Jason Humprhies explains:“At Legendary we’ve been fortunate to work on and restore many rare and desirable muscle cars over the years, but to us it’s important to make sure these cars are set up and run correctly when they leave. But if you buy a car that’s already restored you should really try and make it run at it’s best too. A lot of the time we find cars that look good but run poorly, and a lot of that has to do with a carb that’s not set up right. With a Q-jet like this one, crucial to correct operation is making sure the float level is set correctly and you use the right size primary metering rods. Early Q-jets like this feature single-taper primary rods; these are not interchangeable with those found on 1975 and later carbs. You also need to check operation of the choke pull out which activates the secondaries; many guys think the secondaries are most important, but opening them up too quickly on one of these engines causes the motor to stumble. On a big-torque engine like a Buick 455 with a Q-jet, it’s the primaries that do most of the work. You can find out what the stock specs on the carb are by looking at the carb number, which should be stamped on the top of the bowl above the primary throttle shaft.

Another thing a lot of guys forget is the ignition. You should check the dwell and make sure the points are set before you even try dyno tuning. Another option is going with an electronic ignition like a Crane XR-I or Petronix Ignitor; these feature an electronic pickup that replaces the factory points and condenser. It fits under the stock distributor, so everything still looks factory. It promotes better starting and more even ignition, right from idle through higher rpm and you don’t have to worry about burning points or the condenser because it’s maintenance free.

Another thing that gets overlooked is choosing the right oil. Because most vintage muscle car engines have lots of bottom-end clearance, thicker oils provide better lubrication. A thicker mineral oil like 10W40 tends to work very well to lubricate the moving parts. Also if your engine is freshly rebuilt, it’s a good idea to use an oil with high zinc content for break in. Zinc helps it stick to the moving parts–especially the valvetrain on older cars with flat tapped camshafts–and it helps the valves seat properly. Diesel oils like 10-40s and 10-50s have high zinc content and work very well as a break in oil on these old engines. As for synthetics, you should only use oils that are up to 20 percent synthetic. The problem is that synthetic oils work, but if the car sits for extended periods, the oil doesn’t stay on the cylinder walls, which can cause excessive oil consumption every time you fire it up and run it.”